The present invention relates to an antenna of waveguide type which is particularly suited to receiving/transmitting signals from/to several geostationary satellites and it further relates to a method of manufacturing such an antenna.
Today a multitude of satellites are fixedly placed in the so called geostationary path. Such a satellite is located at a substantially fixed point above the surface of the earth, straightly above a fixed point on the equator. These satellites transmit or forward, in addition to other information, television signals which are intended for private homes, premises or apartments and which are usually received by means of paraboloidal antennas placed directly in the vicinity of the place where the signal is to be used for showing television.
Paraboloidal antennas or aerials, commonly called parabolic reflectors or satellite dishes, of varying sizes are available. In order to distinguish between two satellites which are located at an angular distance of 3xc2x0 from each other as seen from the receiver and which for example transmit using frequencies in the range of 10-12 GHz, a paraboloidal antenna must have a diameter of 60 cm to eliminate mutual interference between signals received from two such satellites. In a paraboloidal antenna having such a size a considerable problem resides in that there is no physical space between the two focal points on which two such adjacent satellites are imaged in order to place the two receiver horns at the focal points. Such receiver horns should have a diameter of 36-42 mm and it appears that the distance between focal points obtained when receiving signals from satellites located so close to each other is significantly smaller than this preferred least diameter of the horns. A paraboloidal antenna suited to receiving signals from satellites having such an angular distance of each other must then be given a larger focus distance, i.e. the distance from each focal point to the center of the paraboloidal antenna must be made larger. Then also all of the paraboloidal antenna must be made significantly larger than the size required for obtaining the signal strength at the focal points which is required for only distinguishing between the signals so that the signals when receiving them will not interfere with each other.
An alternative to paraboloidal antennas comprises antennas of lens character or waveguide type, see e.g. the published International patent application WO 94/11920 A1 and U.S. Pat. No. 2,599,763. When using such an antenna for receiving from two satellites which are located at some angular distance of each other as seen from the receiver, the focal points on which these satellites are imaged are located at the same angular distance as seen from the center of the antenna. However, also in this case the focal distance must be made sufficiently long, in order that there will be sufficient space for the two microwave horns to be located at each other. Since microwave horns for receiving signals having a frequency of for example 11 GHz are less efficient if they have diameters smaller than 40 mm, it is advantageous to place the imaged points, which are obtained when receiving signals from two satellites having an angular distance of 3xc2x0 of each other, at a distance of at least 40 mm from each other. However, the focal distance of the antenna will then be larger than 800 mm.
It is an object of the invention to provide a receiver device for microwave signals from for example satellites allowing simultaneous reception of signals having directions of 2-3xc2x0 from each other in a relatively small antenna.
This and other objects are achieved with a particularly designed antenna which can give a magnification or reduction of the incidence angles of incoming signals.
An antenna of waveguide character suited for receiving/transmitting electromagnetic signals from/to at least two satellites which are fixedly placed at points in the geostationary path has in the common way waveguiding channels. These channels are given such a shape that a separation of the signals is achieved for a shorter focal distance, what for the receiving case will mean a magnification of the angles of incidence. Thereby an increased distance from each other of the focal points for signals from adjacent satellites is obtained. It is also possible to design the waveguiding channels in the antenna so that a reduction of the angles of incidence is obtained if it would be desired.
The characteristic feature of antennas of waveguide type is that an electromagnetic wave passing through such an antenna passes through the antenna in a way similar to that of light passing through an optical lens. In such antennas waveguiding channels are provided which according to the prior art are parallel to the optical axis of the antenna and have varying lengths, diameters and inclinations of wall portions, see the above cited International patent application WO 94/11920 and the prior art described or cited therein. In the cited U.S. patent some channels can be said not to be parallel to the axis but they still work like the other channels being parallel to the axis. In a first embodiment of an antenna of waveguide or lens type considered herein the waveguiding channels are symmetrically placed about the optical axis of the antenna and which, for channels at the same distance of the axis, have the same length, i.e. the antenna is basically rotationally symmetric. In a second embodiment antennas of waveguide guide include waveguiding channels of the same basic type which are not rotationally symmetrically placed and such antennas can also be adapted to give the same effect comprising a magnification or a reduction of the angles of incidence.
The waveguiding channels are generally arranged about an axis of the antenna and produce, when the antenna is used for receiving electromagnetic signals, images of remote objects on focal points located in a focal plane, the true focus of the antenna being the focal point located on the antenna axis. All of the waveguiding channels form angles to the axis which angles are adapted, so that an electromagnetic signal from a remote object arrives to the antenna in a direction forming a first angle to the axis, after passing through the antenna and being refracted therein leaves the antenna in a second angle to the axis different from the first angle. The direction of a signal from a remote object is perpendicular to the flat wavefronts of the signal. After passing through the antenna the signal can obtain substantially spherical wavefronts and the direction of the signal is then defined as the center or symmetry line of the wavefronts. The waveguiding channels have all substantially the same cross-section. They are made as channels having a substantially uniform cross-section over the lengths of the channels except for the entrance and exit regions of the channels which may be tapering to form horn structures. The direction of a channel is given by the center line of the channel, in particular of the region of the channel having the substantially uniform cross-section. The angle of a waveguiding channel can be taken as defined by a straight line connecting the center of an entrance opening of the channel with the center of an exit opening.
The waveguiding channels can be curved and then the center lines thereof e.g. all have a convex polygon shape. Such a center line will generally be located in a plane through the antenna axis. The openings of the waveguiding channels at the exit side, i.e. the parts of the waveguiding channels located close to the exit side, are advantageously directed substantially in a direction towards the true focus of the antenna. Thus said center line of a channel can at the exit side have a direction pointing to the focus. The waveguiding channels, i.e. principally their center lines, can extend substantially along the path of an elementary ray of a signal passing through the antenna, in the refraction of the signal in the antenna.